Foundry binder systems

ABSTRACT

The present invention relates to foundry binder systems forming a curable polyurethane with a catalytically effective amount of a curing catalyst, comprising at least one phenolic resin component comprising at least: a phenolic resin and a solvent of triacetin type and a polyisocyanate component. The invention also relates to foundry mixtures prepared from the binder and an aggregate, to foundry forms such as cores and molds prepared via the no-bake or cold-box processes, and also to the respective processes. The foundry forms obtained by the present invention are especially used for manufacturing metallic components, especially for the casting of metallic components.

The present invention relates to foundry binder systems forming a curable polyurethane with a catalytically effective amount of a curing catalyst, comprising at least one phenolic resin component comprising at least: a phenolic resin and a solvent of triacetin type for the phenolic resin and a polyisocyanate component. The invention also relates to foundry mixtures prepared from the binder and an aggregate, foundry forms such as cores and moulds prepared via processes without baking or in a cold box, and also to the respective processes. The foundry forms obtained by the present invention are especially used for manufacturing metallic components, especially for casting metallic components.

BACKGROUND OF THE INVENTION

A standard process used in the foundry industry for manufacturing metallic components is sand casting. In sand casting, disposable foundry forms, such as moulds and cores, are produced by the shaping and curing of a foundry binder system consisting of a mixture of sand and binder. The binder is used to reinforce the moulds and cores. The steps that follow the curing of the binder are the following:

-   -   the molten metal is cast to fill the cured mould,     -   the cast material is cooled,     -   the cast material thus obtained is removed, and the         corresponding mould is destroyed,     -   the sand is optionally reused in another binder system.

Two of the main processes used, in sand casting to produce moulds and cores are the “no-bake” process and the “cold-box” process. In the no-bake process, a liquid curing agent is mixed with an aggregate, generally sand, and shaped to produce a cured core and/or mould. The no-bake process is based on the room-temperature curing of two or more binder components after they have been combined with sand. The curing of the binder system begins immediately after the addition of a liquid curing agent to all the components, producing a cured mould and/or a core. In the cold-box process, a gaseous curing agent is passed through a shaped compacted mixture so as to form a cured core and/or mould. The term “cold-box process” implies the room-temperature curing of a mixture of binder and sand accelerated with a vapour or gaseous catalyst that is passed through the sand. The binders forming polyurethane, cured with a gaseous tertiary amine catalyst, are often used in the cold-box process to keep the foundry aggregates together in mould or core form as described in American patent U.S. Pat. No. 3,409,579. The binder system forming polyurethane generally consists of a phenolic resin component and a polyisocyanate component that are mixed with sand before compacting and curing to form a foundry binder system.

A person skilled in the art knows that the best use of a solvent is that which is suitable both for the phenolic resin component and for the polyisocyanate component and, if need be, for other binder additives, for example the polyisocyanate curing component.

The oldest class of solvents in this technique is probably that of aromatic hydrocarbon-based compounds, for example benzene, toluene, xylene and ethylbenzene.

Certain particular esters are also known as solvents for the phenolic resin used in polyurethane-forming foundry binder systems. A few examples of these are dioctyl adipate and propylene glycol monomethyl ether acetate (WO 89/07626), dibasic esters (WO 91/09908); ethyl acetate (EP 1 809 456); methyl decanoate, methyl undecanoate and vinyl decanoate (RD425045); 1,2-diisobutyl phthalate, dibasic esters and butyldiglycol acetate (EP 1 074 568), and dialkyl esters (U.S. Pat. No. 55,168, U.S. Pat. No. 4,852,629).

Moreover, another class of esters, i.e. acetic acid diesters such as glyceryl triacetate (or triacetin, RN102-76-1), glyceryl diacetate (or diacetin, RN2539531-7) and glyceryl monoacetate (or monoacetin, RN 26446-35-5), alone or as a mixture, are also known as being able to be used in foundry binder systems, but only as curing agents, for example as described in patents JP 4198531, U.S. Pat. No. 5,602,192, U.S. Pat. No. 5,169,880, U.S. Pat. No. 5,043,412, CS 258845, JP 03018530, U.S. Pat. No. 4,468,359 and U.S. Pat. No. 3920,460. The typical content of triacetin as catalyst in the prior art is 0.5% by weight relative to the weight of the phenolic resin.

INVENTION

The present invention relates to the discovery, not disclosed or suggested in the prior art, that triacetin or mixtures of mono-, di- and triacetin, are solvents that are useful for phenolic resins in polyurethane-forming foundry binder systems, alone or as a mixture with other solvents.

The present invention thus relates to the use of a mixture comprising at least triacetin as solvent that is useful for phenolic resins in polyurethane-forming foundry binder systems, alone or as a mixture with other solvents.

The present invention also relates to a polyurethane foundry binder system that is curable with a catalytically effective amount of a curing catalyst comprising at least:

-   -   A. a phenolic resin component comprising at least:     -   (1) a phenolic resin; and     -   (2) a solvent comprising at least triacetin; and     -   B. a polyisocyanate component.

Another aspect of the present invention relates to foundry mixtures comprising components A and B above with an aggregate, for example sand.

In yet another aspect, the present invention relates to a process for preparing a foundry form by the cold-box process or by the no-bake process, which involves the curing of moulds and cores prepared with the above binder, or the above foundry mixture.

In the cold-box process, the catalyst is in particular a tertiary amine.

The qualities of the triacetin solvent for the phenolic resin according to the present invention are the following, in comparison with the solvents of the prior art:

-   -   compatibility with the phenolic resin,     -   absence of nitrogenous compounds (N2), which contribute towards         the production of gas during casting, thus giving rise to the         appearance of defects in the metallic component,     -   acceleration of the catalytic process for the reaction between         the phenolic resin and the polyisocyanate,     -   absence or low content of OH-, which reacts with polyisocyanate         (for example MDI) and promotes the loss of properties,     -   low hygroscopicity, reducing the possibility of reaction between         water and a polyisocyanate, promoting the loss of properties, or         the generation of gas during casting, thus giving rise to the         appearance of defects in the metallic component,     -   flash point of greater than 120° C., which reduces the         flammability, low content of volatile organic compounds,     -   low emission of smoke; smoke gives rise to defects in the         metallic component,     -   solvability: the viscosity of the resin+solvent mixture is         sufficient to ensure the coating of the sand,     -   very low release of odour,     -   readily biodegradable,     -   non-biocumulative,     -   not classed as carcinogenic,     -   not classed as mutagenic,     -   low aquatic toxicity,     -   virtually non-irritant to the eyes or the skin.

The chemical compounds mentioned in this text, such as triacetin, should be used with care and precaution in the process of the present invention, according to the technical norms.

Solvent

The solvent according to the invention thus comprises at least triacetin. This triacetin may be obtained via a process using crude glycerol, for example the process described in patent application EP 2 272 818. The solvent may be a mixture comprising at least triacetin, and monoacetin and/or diacetin. The solvent may be a mixture comprising at least 80% by weight of triacetin.

Preferentially, the mixture comprises triacetin, monoacetin and diacetin.

In one particular embodiment, the solvent is a mixture of 80% to 95% by weight of triacetin, 5% to 15% by weight of diacetin and less than 5% by weight of monoacetin, relative to the total weight of the said mixture. Triacetin has the formula (AcO)—CH₂—CH(OAc)—CH₂(OAc). Diacetin has the formula (AcO)—CH₂—CH(OH)—CH₂(OAc). Monoacetin has the formula (AcO)—CH₂—CH(OH)—CH₂(OH). In the above formulae, Ac denotes CH₃C(═O). Triacetin known as “industrial triacetin” is a mixture containing from 80% to 95% by weight of triacetin, 5% to 15% by weight of diacetin and less than 5% by weight of monoacetin, relative to the total weight of the said mixture. It is advantageously used as a solvent for the phenolic resin in the foundry binder system according to the invention.

A suitable mixture of solvents for phenolic resins according to the present invention, without excluding any other, concerns triacetin and esters such as those that are known and generally used in this type of application. Examples that may be mentioned include dioctyl adipate and propylene glycol monomethyl ether acetate (WO 89/07626), dibasic esters (WO 91/09908); ethyl acetate (EP 1 809 456); methyl decanoate, methyl undecanoate and vinyl decanoate (RD425045); 1,2-diisobutyl phthalate, dibasic esters and butyldiglycol acetate (EP 1 074 568), and dialkyl esters (U.S. Pat. No. 55168 and U.S. Pat. No. 4,852,629).

A particular suitable mixture of solvents for phenolic resins according to the present invention, without excluding any other, comprises triacetin and a mixture of dimethyl esters, for example the product sold in Brazil under the brand name Rhodiasolv RPDE, comprising dimethyl adipate (RN 627-93-0), dimethyl glutarate (RN1119-40-0) and dimethyl succinate (RN 106-65-0), also used as phenolic resin solvent that is useful in polyurethane-forming foundry binder systems, in particular in the no-bake or cold-box process. Without, however, excluding the others, the appropriate proportions between the triacetin solvent and the said mixture of dimethyl ester ranges from 1:20 to 20:1.

According to the present invention, other phenolic resin solvents, for example aromatic hydrocarbons such as benzene, toluene, xylene, alkylbenzenes such as ethylbenzene, may also be used with triacetin, or with a mixture of solvents comprising triacetin and dimethyl esters.

Phenolic Resin

The phenolic resin component of the present invention is used as a solution of triacetin organic solvent, per se or with cosolvents.

The appropriate phenolic resins are those that are known to a person skilled in the art, which are solid or liquid, but soluble in organic solvents. The amount of solvent used in component A should be sufficient to result in a binder composition that allows uniform coating thereof on the aggregate and a uniform reaction of the mixture. Despite the fact that the concentration of specific solvents varies according to the type of phenolic resin used and its molecular weight, the concentration of solvent in component A in general may be up to 60% by weight of the resin solution, and is typically in the range from 10% to 40% and preferably from 10% to 30%.

A particular phenolic resin used in sand casting according to the present invention is a resol phenolic resin, known under the name resol phenolic resins of benzyl ether type, prepared by reacting an excess of aldehyde with a phenol in the presence of an alkaline catalyst or a metallic catalyst. Without, however, excluding the others, the appropriate phenolic resins are preferably substantially free of water. Examples of phenolic resins used in the binder compositions under consideration are well known to those skilled in the art, such as those described in U.S. Pat. No. 3,485,797. These resins predominantly contain bridges connecting the phenolic nuclei of the polymer, which are ortho-ortho benzyl ether bridges. Generally, they are prepared by reacting an aldehyde and a phenol in an aldehyde/phenol mole ratio of from 1.3:1 to 2.3:1 in the presence of a metal-ion catalyst, preferably a divalent metal ion such as zinc, lead, manganese, copper, tin, magnesium, cobalt, calcium or barium.

As known to those skilled in the art, the phenols used for preparing the resol phenolic resins comprise one or more of the phenols that were used hitherto in the formation of phenolic resins and that are not substituted either on two ortho positions or on one ortho position and on the para position. These unsubstituted positions are necessary for the polymerization reaction. Any of the remaining carbon atoms of the phenolic ring may be substituted. The nature of the substituent may vary widely on condition that the substituent does not seriously interfere with the polymerization of the aldehyde with the phenol on the ortho and/or para position. The substituted phenols used in the formation of the phenolic resins comprise substituted alkylphenols, substituted arylphenols, substituted cycloalkylphenols, substituted aryloxy-phenols and substituted halophenols, these substituents containing from 1 to 26 carbon atoms and preferably from 1 to 12 carbon atoms.

Particular examples of suitable phenols comprise phenol, 2,6-xylenol, o-cresol, p-cresol, 3,5-xylenol, 3,4-xylenol, 2,3,4-trimethylphenol, 3-ethylphenol, 3,5-diethylphenol, p-butylphenol, 3,5-dibutylphenol, p-amylphenol, p-cyclohexylphenol, p-octylphenol, 3,5-dicyclohexylphenol, p-phenylphenol, p-crotylphenol, 3,5-dimethoxyphenol, 3,4,5-trimethoxyphenol, p-ethoxy-phenol, p-butoxyphenol, 3-methyl-4-methoxyphenol and p-phenoxyphenol. Multi-ring phenols such as bisphenol A are also suitable.

The appropriate aldehydes used for reacting with the phenol in order to obtain a resol phenolic resin have the formula R—CHO in which R is a hydrogen atom or a hydrocarbon-based radical containing 1 to 8 carbon atoms. The aldehydes that react with the phenol may comprise one of the aldehydes used hitherto in the formation of phenol resins, such as formaldehyde, acetaldehyde, propionaldehyde, furfuraldehyde and benzaldehyde.

Polyisocyanate

The polyisocyanate component of the binder according to the present invention comprises a polyisocyanate, a polyisocyanate solvent and optional ingredients. The polyisocyanate has a functionality of two or more, preferably from 2 to 5. It may be aliphatic, cycloaliphatic or aromatic, or a hybrid polyisocyanate. The polyisocyanates may also be protected polyisocyanates, polyisocyanate prepolymers and polyisocyanate quasi-prepolymers. Mixtures of polyisocyanates are also covered by the present invention.

The polyisocyanate component of the foundry binder comprises a polyisocyanate, generally an organic polyisocyanate, and an organic solvent, generally comprising aromatic hydrocarbons, such as benzene, toluene, xylene and/or alkylbenzenes, in amounts typically ranging from 0% by weight to about 80% by weight, relative to the weight of the polyisocyanate. Optional ingredients such as release agents and working-life extenders may also be used in the polyisocyanate component.

Representative examples of polyisocyanates used in the present invention are aliphatic polyisocyanates such as hexamethylene diisocyanate, alicyclic polyisocyanates such as 4,4′-dicyclohexylmethane diisocyanate, and aromatic polyisocyanates such as 2,4- and 2,6-toluene diisocyanate, diphenylmethane diisocyanate, and the dimethyl derivatives thereof. Other examples of polyisocyanates are 1,5-naphthalene diisocyanate, triphenylmethane triisocyanate, xylylene, and the methyl derivatives thereof, polymethylenepolyphenyl isocyanates, chlorophenylene 2,4-diisocyanate, etc.

The polyisocyanates are used in concentrations that are sufficient to bring about curing of the resin after passage of the gas or when in contact with the liquid curing catalyst. In general, the ratio between the polyisocyanate and the hydroxyl of the phenolic resin is from 1.25:1 to 1:1.25. The amount of polyisocyanate used is generally from 10% to 500% by weight relative to the weight of the phenolic resin.

The appropriate polyisocyanates are used in particular in liquid from, which may be used in undiluted form, whereas solid or viscous polyisocyanates are used in the form of solutions of organic solvents. Without excluding other possibilities, the adequate solvents for polyisocyanate are AB9 and AB10 (alkylbenzene compounds containing an alkyl substituent that comprises, respectively, 9 and 10 carbon atoms, for example sold under the brand name Solvesso 100).

A person skilled in the art knows that the difference in polarity between the polyisocyanate and the phenolic resins restricts the choice of solvents with which the two components are compatible. This compatibility is necessary in order to achieve a complete reaction and curing of the binder compositions of the present invention. Polar solvents, either of the protic or aprotic type, are good solvents for the phenolic resin, but have limited compatibility with polyisocyanate. Aromatic solvents, although compatible with polyisocyanate, are less compatible with phenol resins. It is thus preferable to use combinations of solvents and in particular combinations of aromatic and polar solvents.

Examples of aromatic solvents for polyisocyanates comprise benzene, toluene, xylene and alkylbenzenes, and mixtures thereof. The aromatic solvents are preferably a mixture of aromatic solvents whose boiling point ranges from 125° C. to 250° C. As cosolvents, the polar solvents should not be extremely polar, which would make them incompatible with the aromatic solvent. The appropriate polar solvents are generally those classed in the prior art as coupling solvents and include furfural, furfuryl alcohol, 2-ethoxyethyl acetate (Cellosolve™ acetate), 2-butoxyethanol (butyl Cellosolve™), diethylene glycol monobutyl ether (butyl Carbitol™), diacetone alcohol and 2,2,4-trimethyl-1,3-diol monoisobutyrate (Texano™). Cellosolve, Carbitol and Texanol are trade names.

The polyisocyanate component B optionally comprises an aromatic hydrocarbon-based compound.

Process

An optional element for a polyurethane-forming foundry binder system is a natural oil. The natural oil is used in the phenolic resin component, the isocyanate component, or in both, in an effective amount that is sufficient to improve the tensile strength of the binder-based foundry forms. This amount is generally between about 1% by weight to about 15% by weight relative to the weight of the isocyanate compound. In general, smaller amounts of natural oil are used in the phenolic resin component, generally about 1% by weight to about 5% by weight, relative to the weight of the phenolic resin. The compatible natural oils are also adequate. A natural oil is considered as being compatible with the organic isocyanate and/or the phenolic resin if the mixture does not separate into two phases at room temperature, and preferably if it does not separate at temperatures between 30° C. and 0° C. Natural oils include unmodified natural oils and also the various known modifications thereof, for example oils thickened by heat, blown with air or oxygen, such as blown linseed oil and blown soybean oil. They are generally classified as ethylenically unsaturated fatty acid esters. The appropriate viscosities of the natural oil range from A to J according to the Gardner Holt viscosity index. The acidity value of the natural oil generally ranges from 0 to 10, as measured by the number of milligrams of potassium hydroxide required to neutralize a sample of 1 gram of natural oil.

As representative examples of the natural oils that are used in the isocyanate component, mention may be made of linseed oil, including refined linseed oil, epoxidized linseed oil, refined linseed oil with alkali, soybean oil, cottonseed oil, RBD (refined, blanched and deodorized) canola oil, refined sunflower oil, tung oil and dehydrated castor oil. As natural oils that are particularly used, mention may be made of purer forms of natural oils that are treated to remove the fatty acids and other impurities, generally consisting of triglycerides and of less than 1% by weight of impurities such as fatty acids and other impurities. As particular examples of these pure natural oils, mention may be made of polymerized linseed oils (PLO) such as supreme linseed oil with an acidity value of about 0.30 and a maximum viscosity of A, and purified soybean oils such as refined soybean oil with an acidity value of less than 0.1 and a viscosity of A to B. As is already known, this increases the tensile forces of foundry forms.

It should be added that the drying oils as described in patent U.S. Pat. No. 4,268,425 may be included in the binder in one of the solvents mentioned herein. These drying oils comprise fatty acid glycerides containing two or more double bonds. In addition, ethylenically unsaturated fatty acid esters such as pine oil esters of polyhydric or monohydric alcohols may be used as drying oils. Generally, the drying oils are used at between about 35% and about 50% by weight of the total amount of solvent.

The binder may also contain a silane, generally added to the phenolic resin component, for example as described in patent U.S. Pat. No. 6,288,139. For example, the silane is added to the phenolic resin component in amounts of 0.01% to 2% by weight relative to the weight of the phenolic resin.

During the preparation of an ordinary foundry form of the sand type, it is common practice to use the aggregate with a relatively large particle size in order to provide a porosity that is sufficient for the foundry form, so as to enable the evacuation of the volatile materials from the form during the casting operation. The term “ordinary foundry forms of sand type” as used herein refers to foundry forms that have a porosity that is sufficient to enable the evacuation of the volatile materials during the casting operation. In general, at least about 80% by weight of the aggregates used for foundry forms have a mean particle size not less than about 50 and at about 150 mesh (Tyler Screen Mesh).

An adequate aggregate used for ordinary foundry forms is silica in which about 70% by weight of the sand consists of silica. Other suitable materials include zircon, olivine, aluminosilicate, sand, chromite sand, etc. Although the best results are often obtained when the aggregate used is dry, it may contain small amounts of moisture.

In the moulding compositions, the aggregate constitutes the main constituent, the binder being present in a relatively small amount. In foundry applications of the sand type, the amount of binder generally does not exceed about 10% by weight and is frequently in the range from 0.5% to 7% by weight relative to the weight of the aggregate, in particular 1.0% to 1.8%.

The binder compositions are preferably provided as a two-packet system with the phenolic resin component in one packet and the polyisocyanate component in the other. Usually, the phenolic resin component is mixed with the aggregate, and the polyisocyanate component is then added. The methods for distributing the binder over the aggregate particles are well known to those skilled in the art.

Another aspect of the present invention relates to processes for obtaining forms by sand casting, using the novel binder system, which is moulded in the desired shape, such as a mould or core, and cured. Curing via the cold-box process is performed by passing a volatile tertiary amine, for example triethylamine, 1-dimethylamino-2-propanol (DMA-2P), monoethanolamine or dimethylaminopropylamine (DMAPA), through the form in the mould as described in U.S. Pat. No. 3,409,579. Curing via the no-bake process is performed by mixing a liquid amine curing catalyst into the foundry binder system, which is then shaped, and cured.

In accordance with the binder system according to the invention, the term “catalytically effective amount of a curing catalyst” means a concentration of the catalyst preferentially between 0.2% and 5.0% by weight of the phenolic resin.

The useful liquid amine curing catalysts have a pKb value generally of the order of 7 to 11. Particular examples of these amines that may be mentioned include 4-alkylpyridines, isoquinoline, arylpyridines, 1-methylbenzimidazole and 1,4-thiazine. A liquid tertiary amine that is particularly used as catalyst is an aliphatic tertiary amine such as tris(3-dimethyl-amino)propylamine. In general, the concentration of the liquid amine catalyst ranges from 0.2% to 5.0% by weight of the phenolic resin, in particular from 1.0% to 4.0% by weight and more particularly from 2.0% to 3.5% by weight relative to the weight of the polyether polyol. Catalysts such as triethylamine or dimethyl-ethylamine are used in a range of from 0.05% to 0.15% by weight relative to the weight of the binder.

Specific language is used in the description so as to facilitate the understanding of the principle of the invention. It should be understood, however, that no limitation of the scope of the invention is envisaged by the use of this specific language. Modifications, improvements and perfections may especially be envisaged by a person skilled in the technical field concerned on the basis of his own general knowledge.

The term “and/or” includes the meanings “and” and “or”, and also all the other possible combinations of elements connected with this term.

Other details or advantages of the invention will emerge more clearly in the light of the examples given below, purely for indicative purposes.

EXPERIMENTAL SECTION

The example that follows represents a particular embodiment of the invention, without any limiting nature, as described in the set of claims presented later.

The solubilizing power of the solvents tested in this example was determined by simulation using the Solsys® software (Rhodia). It is based on the theory of solubility parameters and the Hansen three-dimensional system. Specifically, as is known to those skilled in the art, the cohesion energy parameters most widely used for the characterization of solvents are those developed by Hansen (for example in the book “Hansen Solubility Parameters: A user's handbook” Hansen, Charles Second Edition 2007 Boca Raton, FL, United States. CRC Press). There are three figures which, together, are known as the HSP. They fully describe the way in which a solvent behaves relative to that which is dissolved if their HSP values are known or can be estimated:

-   -   δD—the dispersion energy of the bonds between the molecules     -   δP—the energy of the intermolecular dipolar force between         molecules     -   δH—the energy of hydrogen bonds between molecules.

Hansen demonstrated that the substances are characterized by δD, δP and δH.

The technique for determining the solubility parameters D, P and H of a substance, namely of a phenolic resin in this example, consists in testing the solubility of the said substance in a series of pure solvents that belong to different chemical groups (for example hydrocarbons, ketones, esters, alcohols and glycols). The evaluation is made by considering the solvents that fully or partially dissolve or that do not dissolve the substance to be dissolved. The Solsys® software makes it possible to determine the solubility volume of the substance and, as a consequence, it makes it possible to determine the best solvent for dissolving the substance. The solubility volume is represented by a sphere (three-dimensional system) whose centre corresponds to a “normalized distance” equal to 0 and reflects the solubility maximum. All the points located on the surface of the sphere correspond to a “normalized distance” equal to 1 and reflect the solubility limit. The sphere is represented on a graph whose axes correspond to δD, δP and δH. The solubility of the resin in the solvent will be proportionately greater the closer the solubility volume is to the centre (normalized distance equal to 0). Beyond the surface of the sphere (normalized distance equal to 1), the resin is no longer soluble in the solvent.

The “normalized distance” values are used to evaluate the solubilizing power of a substance, namely of the phenolic resin in the present case, in a solvent. The closer the value of the normalized distance to 0, the more soluble the resin in the solvent.

Table I below shows the solubility parameter values for a commercial mixture of dimethyl esters, relative to triacetin with a purity of greater than 99.5% and industrial triacetin. The normalized distances were obtained for a resol phenolic resin.

TABLE I Rhodiasolv Triacetin Solubility RPDE (*) (purity Industrial parameters (comparative) >99.5%) triacetin (**) δD 16.87 16.5 16.5 δP 4.87 4.5 8.7 δH 10.02 9.1 11.5 Normalized distance 0.13 0.15 0.08 (*) 62% by weight of dimethyl glutarate, 23% by weight of dimethyl succinate, 15% by weight of dimethyl adipate. (**) About 86% by weight of triacetin, 10% by weight of diacetin, 4% by weight of monoacetin.

The general formulae of the solvents RPDE, triacetin and industrial triacetin are given below.

As is seen, the solubility parameter values for the mixture of commercial dimethyl ester solvents are similar to those obtained for triacetin. This means that the solvents are partially or totally interchangeable, for the majority of applications, in particular as a mixture, which depends on the commercial conditions. The normalized distances are also very similar, especially for the solubility in RPDE and triacetin of purity greater than 99.5%.

Furthermore, it was demonstrated that industrial triacetin has a greater solubilizing power. Specifically, the normalized distance obtained is less than that obtained with RPDE and triacetin alone, which makes it possible to reduce the amount of solvent to dissolve the phenolic resin, when compared with Rhodiasolv RPDE and triacetin, without loss of performance.

The presence of hydroxyl groups (—OH) in monoacetin and diacetin justifies the increase in polarity (δP value—Table I) and of the solubilizing power of industrial triacetin. By respecting the concentration range of triacetin in industrial triacetin (from 80% to 95% by weight), no adverse reaction is observed between the free —OH groups and the polyisocyanate.

With the information given here, a person skilled in the art is capable of reproducing the invention in various ways, but for the same purpose to achieve similar results. These equivalent embodiments are also covered by the claims below. 

1. Foundry binder system forming a curable polyurethane with a catalytically effective amount of a curing catalyst, comprising at least: A. a phenolic resin component comprising at least: (1) a phenolic resin: and (2) a solvent comprising at least triacetin: and B. a polyisocyanate component
 2. A foundry binder system as defined by claim 1, in which the triacetin is obtained from a process using crude glycerol.
 3. A foundry binder system as defined by claim 1, in which the solvent is a mixture comprising at least 80% by weight of triacetin.
 4. A foundry binder system as defined by claim 1, in which the solvent is a mixture comprising 80% to 95% by weight of triacetin, 5% to 15% by weight of diacetin and less than 5% by weight of monoacetin, relative to the total weight of the mixture.
 5. A foundry binder system as defined by claim 1, in which the said component A also comprises a solvent that is a mixture of dimethyl esters, especially dimethyl adipate, dimethyl glutarate and dimethyl succinate.
 6. A foundry binder system as defined by claim 5, in which the ratio of the solvent comprising at least triacetin and of the said mixture of dimethyl esters ranges from 1:20 to 20:1.
 7. A foundry binder system as defined by claim 1, in which the said component A also comprises one or more aromatic hydrocarbon-based solvents.
 8. A foundry binder system as defined by claim 1, in which the solvent comprising at least triacetin represents from 10% to 30% by weight relative to the weight of component A.
 9. A foundry binder system as defined by claim 1, in which the said phenolic resin is a resol phenolic resin.
 10. A foundry binder defined by claim 1, in which the said polyisocyanate component comprises a polyisocyanate and a polyisocyanate solvent,
 11. A foundry binder system as defined by claim 10, in which the said polyisocyanate solvent is a mixture of aromatic hydrocarbons, chosen especially from benzene, toluene, xylene and alkylbenzenes, and mixtures thereof.
 12. Foundry mixture comprising a polyurethane-forming binder system as defined by claim 1, and an aggregate,
 13. A foundry mixture as defined by claim 12, in which the said aggregate is sand.
 14. Process for preparing a foundry form via the cold-box process, in which the binder is as defined by claim 1, and the curing catalyst is a gaseous amine.
 15. Process for preparing a foundry form via the no-bake process, in which the binder is as defined by claim 1, and the curing catalyst is a liquid amine.
 16. Foundry form prepared via the process as defined by claim
 14. 17. Use of a mixture comprising at least triacetin as a solvent that is useful for phenolic resins in polyurethane-forming foundry binder systems, alone or as a mixture with other solvents. 